/*
 * Copyright MITRE
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.iff.infra.util.servlet;

import org.apache.http.NameValuePair;
import org.apache.http.client.utils.URIUtils;
import org.apache.http.client.utils.URLEncodedUtils;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.net.URI;
import java.net.URISyntaxException;
import java.util.LinkedHashMap;
import java.util.List;
import java.util.Map;
import java.util.regex.Matcher;
import java.util.regex.Pattern;

/**
 * A proxy servlet in which the target URI is templated from incoming request parameters. The
 * format adheres to the <a href="http://tools.ietf.org/html/rfc6570">URI Template RFC</a>, "Level
 * 1". Example:
 * <pre>
 *   targetUri = http://{host}:{port}/{path}
 * </pre>
 * --which has the template variables.  The incoming request must contain query args of these
 * names.  They are removed when the request is sent to the target.
 */
@SuppressWarnings({ "serial" })
public class URITemplateProxyServlet extends ProxyServlet {

	/* Rich:
	* It might be a nice addition to have some syntax that allowed a proxy arg to be "optional", that is,
	* don't fail if not present, just return the empty string or a given default. But I don't see
	* anything in the spec that supports this kind of construct.
	* Notionally, it might look like {?host:google.com} would return the value of
	* the URL parameter "?hostProxyArg=somehost.com" if defined, but if not defined, return "google.com".
	* Similarly, {?host} could return the value of hostProxyArg or empty string if not present.
	* But that's not how the spec works. So for now we will require a proxy arg to be present
	* if defined for this proxy URL.
	*/
	protected static final Pattern TEMPLATE_PATTERN = Pattern.compile("\\{(.+?)\\}");
	private static final String ATTR_QUERY_STRING = URITemplateProxyServlet.class.getSimpleName() + ".queryString";

	protected String targetUriTemplate;//has {name} parts

	@Override
	protected void initTarget() throws ServletException {
		targetUriTemplate = getConfigParam(P_TARGET_URI);
		if (targetUriTemplate == null)
			throw new ServletException(P_TARGET_URI + " is required.");

		//leave this.target* null to prevent accidental mis-use
	}

	@Override
	protected void service(HttpServletRequest servletRequest, HttpServletResponse servletResponse)
			throws ServletException, IOException {

		//First collect params
		/*
		 * Do not use servletRequest.getParameter(arg) because that will
		 * typically read and consume the servlet InputStream (where our
		 * form data is stored for POST). We need the InputStream later on.
		 * So we'll parse the query string ourselves. A side benefit is
		 * we can keep the proxy parameters in the query string and not
		 * have to add them to a URL encoded form attachment.
		 */
		String queryString = "?" + servletRequest.getQueryString();//no "?" but might have "#"
		int hash = queryString.indexOf('#');
		if (hash >= 0) {
			queryString = queryString.substring(0, hash);
		}
		List<NameValuePair> pairs;
		try {
			//note: HttpClient 4.2 lets you parse the string without building the URI
			pairs = URLEncodedUtils.parse(new URI(queryString), "UTF-8");
		} catch (URISyntaxException e) {
			throw new ServletException("Unexpected URI parsing error on " + queryString, e);
		}
		LinkedHashMap<String, String> params = new LinkedHashMap<String, String>();
		for (NameValuePair pair : pairs) {
			params.put(pair.getName(), pair.getValue());
		}

		//Now rewrite the URL
		StringBuffer urlBuf = new StringBuffer();//note: StringBuilder isn't supported by Matcher
		Matcher matcher = TEMPLATE_PATTERN.matcher(targetUriTemplate);
		while (matcher.find()) {
			String arg = matcher.group(1);
			String replacement = params.remove(arg);//note we remove
			if (replacement == null) {
				throw new ServletException("Missing HTTP parameter " + arg + " to fill the template");
			}
			matcher.appendReplacement(urlBuf, replacement);
		}
		matcher.appendTail(urlBuf);
		String newTargetUri = urlBuf.toString();
		servletRequest.setAttribute(ATTR_TARGET_URI, newTargetUri);
		URI targetUriObj;
		try {
			targetUriObj = new URI(newTargetUri);
		} catch (Exception e) {
			throw new ServletException("Rewritten targetUri is invalid: " + newTargetUri, e);
		}
		servletRequest.setAttribute(ATTR_TARGET_HOST, URIUtils.extractHost(targetUriObj));

		//Determine the new query string based on removing the used names
		StringBuilder newQueryBuf = new StringBuilder(queryString.length());
		for (Map.Entry<String, String> nameVal : params.entrySet()) {
			if (newQueryBuf.length() > 0)
				newQueryBuf.append('&');
			newQueryBuf.append(nameVal.getKey()).append('=');
			if (nameVal.getValue() != null)
				newQueryBuf.append(nameVal.getValue());
		}
		servletRequest.setAttribute(ATTR_QUERY_STRING, newQueryBuf.toString());

		super.service(servletRequest, servletResponse);
	}

	@Override
	protected String rewriteQueryStringFromRequest(HttpServletRequest servletRequest, String queryString) {
		return (String) servletRequest.getAttribute(ATTR_QUERY_STRING);
	}
}
